The present invention pertains to a two-stage supercharging device with a high-pressure turbine and low-pressure turbine for internal combustion engines.
A modern internal combustion engine is usually equipped with a two-stage supercharging device. A first stage of the supercharging device is a high-pressure stage, the second stage a low-pressure stage. The high-pressure stage comprises a high-pressure turbine and a high-pressure compressor. The low-pressure stage comprises a low-pressure turbine and a low-pressure compressor. The high-pressure stage and the low-pressure stage of the supercharging device are each provided with a rotor assembly, which, in the one case, comprises a high-pressure turbine wheel, a shaft, and a high-pressure compressor wheel and, in the other case, a low-pressure turbine wheel, a shaft, and a low-pressure compressor wheel. During operation, an exhaust gas stream flows from the internal combustion engine through an exhaust gas line into the high-pressure turbine and then through an exhaust gas channel into the low-pressure turbine. In the upper load ranges of the internal combustion engine, the exhaust gas stream is so strong that the rotor assembly of the high-pressure stage can rotate so fast that it would be damaged by the centrifugal forces which develop. To prevent this, the main exhaust gas stream is divided at a branching point in the exhaust gas line into a primary exhaust gas stream and a secondary exhaust gas stream. The primary exhaust gas stream flows into the high-pressure turbine, whereas the secondary exhaust gas stream flows through a bypass device which takes it around the high-pressure turbine. In the simplest case, the bypass device comprises a branch line branching off from the exhaust gas line at the branching point and a shutoff valve arranged in this branch line. In the upper load range, therefore, the secondary exhaust gas stream is branched off from the main exhaust gas stream into the branch line upstream of the high-pressure turbine, is regulated by the shutoff valve, and is conducted into the exhaust gas channel downstream of the high-pressure turbine. As a result, the primary exhaust gas stream flowing into the high-pressure turbine is reduced, and the rotor assembly of the high-pressure stage is therefore protected from damage. The secondary exhaust gas stream flowing through the bypass device around the high-pressure turbine rejoins the primary exhaust gas stream flowing out of the high-pressure turbine in the exhaust gas channel and then flows into the low-pressure turbine. The bypass device together with its structural attachment to the exhaust gas channel is the primary focus of the invention and is described below in greater detail.
Examples of various designs for this structural attachment are known from the prior art. The publication DE 10 2007 046 667 A1, for example, shows an attachment in the form of a ring-shaped channel extending around the exhaust gas channel. According to the description, the secondary exhaust gas stream flows into this ring-shaped channel, becomes uniformly distributed in it, and then flows radially into the exhaust gas channel. In the description, the basic problems associated with the conditions under which one fluid flows into another are already discussed, but an optimal solution is not yet implemented here. The disadvantage of this publication is that, as the secondary exhaust gas stream is flowing radially inward into the exhaust gas channel, strong vortices develop at the edge of the ring-shaped channel. These are associated with a considerable loss of pressure, and a long exhaust gas channel is necessary to calm the reunited exhaust gas stream to ensure that it flows uniformly into the downstream low-pressure turbine. The pressure loss means a considerable loss of efficiency for the low-pressure turbine. The ring channel arranged downstream from the high-pressure turbine and the long exhaust gas channel lead to the need for a large amount of space to accommodate the components. A loss of efficiency and the need for a large amount of space are unfavorable for an internal combustion engine.
Another prior art is known from EP 1 710 415 A1. The bypass device comprises here the branch line, the shutoff valve, a connecting flange, and the ring-shaped channel. The secondary exhaust gas stream branches off into the branching line upstream from the high-pressure turbine. To regulate the secondary exhaust gas stream, the shutoff valve is installed in the branch line. The secondary exhaust gas stream enters the ring channel through a connecting flange, the channel being arranged in the form of a spiral extending around the exhaust gas channel. The primary and secondary exhaust gas streams are conducted separately up to a point directly upstream of the low-pressure turbine wheel. The disadvantage of this is that the primary and secondary exhaust gas streams cannot recombine upstream of the low-pressure turbine, which leads to radial nonuniformity of the inflow into the low-pressure turbine wheel. In conjunction with the swirling of the secondary exhaust gas stream caused by the spiral shape of the ring-shaped channel, this results in poor efficiency of the low-pressure turbine. An additional disadvantage is the requirement for a large amount of space and a large number of separate components for the layout of the bypass device in the branch line and its shutoff valve; the same applies to the ring channel and its separate connecting flange.